A digitized representation of a continuous-tone image, where gray level or color of each pixel is represented by a continuum of values (e.g. between 0 and 1), may be rendered on a device, such as a printer, capable of producing a limited set of tone levels. Digital halftoning is a process for performing the rendering of the digitized representation with such a limited set. The digitized representation, referred to as a digital halftone or digital halftone representation, may include an array of pixels each having a value of each pixel indicating the tone level of ink or toner deposited by the printer on a corresponding location on a substrate or medium.
The output image produced by the halftoning process may be referred to as a rendered halftone. In the case of binary printing, a limited set of tone levels may represent the presence or absence of a printed dot. In the case of multi-level printing, the limited set of tone levels may include a limited set of dot sizes. In the case of polychromatic binary or multi-level printing, the halftoning process may be applied separately for each color of ink or toner. Halftoning may produce an acceptable image due to characteristics of the human visual system (HVS). The HVS may be modeled as applying a low-pass spatial filter to a perceived image. Thus, an unmagnified rendered halftone may be perceived by a human observer as a continuous-tone image.
The transfer of ink or toner from a reservoir to a printable medium or substrate (e.g. paper) may be a complex process. In particular, an electro-photographic (EP) printing process may include several electrostatic transfers of toner particles. For example, in an EP offset printing process, toner may be transferred from a storage container or reservoir to a latent image on a drum, from the drum to a transfer blanket, and from the transfer blanket to the printing medium. Each transfer may introduce distortion, blurring or other inaccuracies, such that the final printed image may be a perceptibly distorted version of the original digital image. The amount of distortion of a printed dot may be influenced by the presence or absence of neighboring dots. The amount of distortion may vary from printer to printer, and may vary over the lifetime of the printer due to, for example, printer component aging or replacement.